I am in the process of moving down (in speed that is) to a 40 foot Marine Trader Labelle. At the sea trials the engines did not meet the recommended full max. rpm of 2500. The motors are 212 hp Sabre Marine 6 cylinder, turbo(ed). The PO had replaced the original (at least when he bought the boat) 3-blade props with 4-blade ones (D 24 P20 ), which I have. His thoughts were he was buying the boat for the Great Loop and he would get better efficiency at 8 knots with this arrangement. I have the 3 blade props. I plan on doing much more extensive cruising where efficiency at a higher speed would be welcomed. My experience with other machinery is that they are more efficient if run at their "sweet spot". However I have no clue as where it is at this time.
So after all this my question is do I change the props. Here is the performance numbers at the trials.

It is a fairly common, but misguided practice IMO, to over prop a boat several hundred rpms in the expectation of lower fuel consumption at lower rpms. That is what seems to have happened with your boat.

Since you plan to run it at higher rpms than 1,400 for 8 kts, I would go back to the original 3 blade props and make sure the engines hit 2,500 at wot.

Start by calibrating your tach with a handheld device, then do the trials at slack tide in calm water with no wind. If you have to run inlets against the tide then I would want max rpm's and speed. I suspect the 3 blade will give you higher speed at top end.

If strictly cruising then I would probably pick up some digital flow meters on EBay and figure out what most efficient settings are for various conditions. I'm guessing the 4 blades would give you better efficiency at the cost of losing some top end speed. You could always repitch the 4 blades by reducing pitch by an inch or two to get them to max rpm but then you sacrifice some efficiency.

SoWhat,
I’m guessing the 3 blade will give better efficiency because the 4 blade will not have enough pitch and too much blade area. Top speed may depend on the same variables.
But I see you’ve got a 24x20. That would be ok (probably) but when you depitch it to get propper rpm you’ll likely wind up w 24x18. Then the three blade may be better.
Check w Michigan Propeller or find a good local prop guy and feed them w all the correct variables.

Most people moving from a faster boat to a larger slower boat are unhappy with the speed realities of a big boat. They look for ways to wring out an extra knot or two. If your plan is to run at 2500 rpm over long periods, with a turbo diesel, you will shorten the life of the engine.
Heat (and dirty oil) is the enemy of engine lifespan. At full hp, the internal temps created will be slowly wearing away the rings, pistons, cylinder walls and valves. Most experienced operators of small turbo diesels run them at 80% of hp except for short periods of WOT. The difference is usually double the lifespan. The goal of most makers of small yacht diesels is to make it thru the warranty period. After that to sell parts or another engine. Few yacht diesels, if any, have all the expensive metal alloys that are used in large commercial diesels. A small turbo diesel run hard may not have the lifespan of a good gas engine.

Shrew and NBS,
Probably not saving that much fuel except over a long period of time.

Lepke wrote;
“Most experienced operators of small turbo diesels run them at 80% of hp except for short periods of WOT. The difference is usually double the lifespan. The goal of most makers of small yacht diesels is to make it thru the warranty period. After that to sell parts or another engine. Few yacht diesels, if any, have all the expensive metal alloys that are used in large commercial diesels. A small turbo diesel run hard may not have the lifespan of a good gas engine.“

I agree but think you’ve over stated it. 80% to WOT isn’t that much when you go from one to the other if you’re not overpropped. So many are though you could be right. I don’t think it’s rpm that wears an engine out. It’s load. Just above the middle of the stroke it’s how hard the piston is pushing sideways against the cylinder wall and how much lube oil is there to protect it .. or keep them apart.

But if overpropped you could be very correct. And I agree 80% should be fine for an industrial engine.

Sorry it took so long to get back to this subject, working on getting the papers signed etc. My concern is not so much as speed as it is economy. When we were doing survey at the 2100/2200 rpm the engine temp went up quire rapidly which led me to believe there was an issue with the props. I recognize there is a big difference in go fast boats and what I am buying however as my son-in-law said "why are you in a hurry?". At my age I decided I am not. Looking forward to having time to look at the scenery. Thanks to all. Tuesday the 4th is the big day!!!

It is a mistake to assume that over proping an engine will shorten the engines life. That assumption doesn't take into account how the engine is used. My boat is over propped simply because a 4:1 gear doesn't exist for my engine with a down angle. The engine diagnostics show at my normal cruising RPM, the engine is producing less than half of the allowable HP for that RPM. The dealer's comment was," it will never wear out at that load percentage ". Now consider the alternative, if the engine was correctly propped and I ran it wide open all the time, how long do you think it would last? Life expectancy is more about how hard you run an engine. Finally, consider a slow speed generator. They are set up to optimize maximum load at a much lower RPM than the motor was designed for. As long as you don't exceed the load for a given RPM, you can certainly optimize the load for that RPM.

While there may be some small savings in fuel from over propping, it's pretty much insignificant. There can be a significant difference in noise levels though.

Ted

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Most people moving from a faster boat to a larger slower boat are unhappy with the speed realities of a big boat. They look for ways to wring out an extra knot or two.

An autopilot helps overcome the tedium of steering boats at slower speeds than you may be used to. When the kids ask "Are we there yet?" you can look up from the NYTimes on your iPad Poort and announce "OMG, we are!"

I'm also in the camp that says over propping only overloads the engine if you run it overloaded. At substantially less than full rpm, there is a large gap between max engine torque and prop absorption. That gap represents a light load on the engine. In normal circumstances there may not be much economy difference, but there will surely be a noise difference. If you overprop enough to close that gap at normal cruising rpm, then yes, the engine will not last as long - but that is a very extreme case of over propping for most trawlers.

Some engines are more susceptible to damage from overproping/overloading, the Cummins B series being a prime example.

If you overrprop to make the engine produce closer to it’s rated output at a low RPM, you will be significantly overloaded at higher cruise RPM’s, and this wil reduce life, if you actually run at those higher rpm’s

In a full displacement hull boat you might never run at the higher rpm settings, but in a Semi Displacement boat you probably will.

Frankly I see very little to gain by intentionally overpropping, and quite a bit to loose.

Some engines are more susceptible to damage from overproping/overloading, the Cummins B series being a prime example.

If you overrprop to make the engine produce closer to it’s rated output at a low RPM, you will be significantly overloaded at higher cruise RPM’s, and this wil reduce life, if you actually run at those higher rpm’s

In a full displacement hull boat you might never run at the higher rpm settings, but in a Semi Displacement boat you probably will.

Frankly I see very little to gain by intentionally overpropping, and quite a bit to loose.

While you would not want to overprop an engine and then try to run it at near maximum RPM, overpropping by definition should mean giving up an RPM range that you will never uses. In most overpropping examples on this forum, the boat can't plane either because of hull style or lack of HP. In essence, the owner knows or should know what his reasonable maximum attainable speed is, and is now optimizing the RPM range to match the realistic speed range. Clearly the owner and or operator need to understand the concepts involved and new lower RPM limit.

Ted

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Blog: mvslowhand.com
I'm tired of fast moves, I've got a slow groove, on my mind.....
I want to spend some time, Not come and go in a heated rush....."Slow Hand" by The Pointer Sisters